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Message-ID: <20231116141951.GE18748@redhat.com>
Date: Thu, 16 Nov 2023 15:19:51 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: David Howells <dhowells@...hat.com>
Cc: Marc Dionne <marc.dionne@...istor.com>,
"David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>,
Al Viro <viro@...iv.linux.org.uk>,
Chuck Lever <chuck.lever@...cle.com>,
linux-afs@...ts.infradead.org, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rxrpc_find_service_conn_rcu: use read_seqbegin() rather
than read_seqbegin_or_lock()
On 11/16, David Howells wrote:
>
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> > So do you agree that
> >
> > - the usage of read_seqbegin_or_lock/need_seqretry in
> > this code makes no sense because read_seqlock_excl()
> > is not possible
>
> Not exactly. I think it should take a lock on the second pass.
OK, then how about the patch below?
Again, I'd prefer to change the semantics/prototype of need_seqretry()
to enforce the locking on the 2nd pass "automatically", but a) this
needs more discussion and b) I can't do this before I update the users
which use read_seqbegin_or_lock/need_seqretry incorrectly. So lets
discuss this later.
Oleg.
--- a/net/rxrpc/conn_service.c
+++ b/net/rxrpc/conn_service.c
@@ -25,7 +25,7 @@ struct rxrpc_connection *rxrpc_find_service_conn_rcu(struct rxrpc_peer *peer,
struct rxrpc_conn_proto k;
struct rxrpc_skb_priv *sp = rxrpc_skb(skb);
struct rb_node *p;
- unsigned int seq = 0;
+ unsigned int seq = 1;
k.epoch = sp->hdr.epoch;
k.cid = sp->hdr.cid & RXRPC_CIDMASK;
@@ -35,6 +35,7 @@ struct rxrpc_connection *rxrpc_find_service_conn_rcu(struct rxrpc_peer *peer,
* under just the RCU read lock, so we have to check for
* changes.
*/
+ seq++; /* 2 on the 1st/lockless path, otherwise odd */
read_seqbegin_or_lock(&peer->service_conn_lock, &seq);
p = rcu_dereference_raw(peer->service_conns.rb_node);
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